It is known that the iridoid compound derived from the gardenia fruit reacts with a primary amino group-containing compound and, upon oxidation polymerization of the resulting reaction product, a blue color dye is produced, as disclosed in Japanese Patent Publication No. 57-14781 and Japanese Patent Unexamined Publication (Kokai) No. 63-47167. Also, Japanese Patent Unexamined Publication (Kokai) No. 63-157981 discloses that the iridoid compound reacts to form crosslinks between proteins or between an enzyme protein and an amino group-containing carrier and, upon oxidation polymerization, it forms a stable crosslinking structure. Further, the above-described ability to form a crosslinking structure can be utilized for improving the physical properties of primary amino group-containing compounds such as proteins, for example, heat-resistance, water-resistance, acid-resistance, and also is expected to have a wide variety of applications.
However, the crosslinked product obtained by using the iridoid compound has a blue color due to its chemical structure, thus limiting its applications. For example, the blue color produced in crosslinked products precludes their use in most foodstuffs, because types and quantities of foods having a cold color, in particular, a blue color, are relatively small.
Thus, it has long been desired to develop a method for decolorizing the blue color dye produced by crosslinking a primary amino group-containing compound such as proteins with an iridoid compound, for example, genipine, while maintaining the crosslinked structure of the resulting crosslinked primary amino group-containing compound.
As a result of extensive studies, the present inventors found that the blue color can be decolorized by treating the crosslinked product obtained with an iridoid compound, for example, genipine with an oxidizing agent, a reducing agent or a reductone such as ascorbic acid (or isoascorbic acid), and on that basis completed the present invention.